In the manufacture of impact grades of polystyrene, styrene solutions of up to about 10 weight percent polybutadiene are prepared in a dissolving vessel and filtered during or prior to transfer to a polymerization reactor. Solid bales (75 lb or 34 kg) of polybutadiene are cut into small pieces for rapid dissolution, prior to placement in a dissolver. Cold styrene/polybutadiene slurry is heated typically to 140.degree.-180.degree. F. to increase the dissolution rate. After a few hours, i.e. about 1-3 hours or more (grades with the highest rubber content will take longer) the solution is filtered to remove residual polybutadiene gels. These gels swell in styrene, but are not soluble and if not removed will produce visible defects in extruded impact polystyrene sheet. They are not uncommonly as large as 1000 microns in the swollen state.
During polymerization of the styrene solution, conditions are closely controlled to minimize new gel formation because the concentration of gels in a 10-mil extruded sheet of prime product is limited to a specified maximum. Gels may be formed either from styrene homopolymerization or from grafting on the polybutadiene or both. Considerable effort is expended to assure gel removal before polymerization because neither type of gel can be removed after polymerization.
Prior to the present invention, a typical commercial facility would employ a pair of horizontal-plate filters having orifices of up to 1/2-inch installed in parallel and fitted with one sheet of grade B and one sheet of grade R filter paper for each of 32 plates. In the past, when the pressure drop across the one filter in use reached 50 psi (344 kPa), or the filtration time increased to about 45 minutes, it became necessary to replace the plugged filter paper. Filter changes have been required about every 200 batches with solutions of rubber containing about 10% vinyl groups and about 35% cis groups. Rubbers of very low vinyl content and high (about 98%) cis content are even more likely to contain gels and cause filtering problems. Even with long dissolution times, filtration of only two batches or less of the high-cis solution could require a filter change.